Metallic or ceramic coatings are commonly deposited using a vacuum process such as physical vapor deposition (PVD) or plasma assisted chemical vapor process (PACVD). In either case, a vacuum chamber is used to generate plasma at low gas pressures, typically from a few millitorr to a few hundred torr. In these conditions, the plasma may be generated relatively easily. Coatings may then be deposited on parts positioned within the vacuum chamber by introducing the components of the coating within the chamber. Although the coating quality of the vacuum-based process is quite high, having to perform the process inside a vacuum chamber system is relatively cumbersome. For some applications, vacuum deposition in a chamber is nearly impossible when treating large structures, such as an aircraft wing or the impeller of a ship, or portions thereof. An atmospheric pressure (AP) plasma deposition process, which does not require a chamber to regulate atmospheric pressure and allows processing of relatively large components, becomes attractive and practical.
One such atmospheric pressure plasma deposition process includes plasma spray. In simplistic form, the plasma spray system includes an insulated housing, a cathode positioned within the housing, an anode surrounding a portion of the cathode providing a nozzle, an arc gas feed, a powder and carrier gas feed, and electrical and water connections. The plasma spray system is operated in relatively high power DC mode of a few hundred kilowatts to ignite the arc gas and achieve a flame temperature in the range of 6,650° C. to 11,000° C. so that metallic powder used in the coating may be melted and deposited on a substrate. In the process the metallic powder is propelled at speeds of 240 to 550 m/s. In embodiments, the cathode is formed of tungsten and the anode of copper, the arc gas includes argon or nitrogen, and the carrier gas may include hydrogen. The housing, anode, and, optionally, the cathode may be water cooled. Overheating of substrates being coated is minimized by rotating or cooling samples or moving samples quickly away from the plasma source. Due to the high power DC operation mode, the resultant coating is relatively thick, typically in the range of 0.1 to 5 mm, and generally porous.
Thermal spray processes, such as high velocity oxygen fuel spray, may also be used to deposit coatings, even though the process does not utilize plasma, the process utilizes relatively high operating temperatures in the range of 3000° C. Lower temperature processes, such as cold spray processes that operate at about 900° C., accelerate relatively fine particles of less than 20 micrometers. However, deposition efficiency is relatively low.
Accordingly, room remains for the improvement of plasma coating systems. Systems that form coatings at lower process temperatures and provide coatings that exhibit relatively low porosity are particularly desirable.